1. Field of the Invention
The present invention relates to a method of efficiently executing cell search of W-CDMA (Wideband CDMA) to reduce power consumption.
2. Description of the Related Art
W-CDMA related to the third generation mobile phones is a successor to the second generation mobile phones which mainly adopts TDMA (Time Division Multiple Access) such as PDC and GSM. It is characterized in that on a transmission side, a narrow band signal is spread into a wideband by a spread code assigned to each base station and transmitted, and is inversely spread on a reception side to reduce effects of noise, as well as enabling transmission of a large volume of data.
W-CDMA receiver has a plurality of RAKE receivers in order to prevent multi-path fading which is a phenomenon that radio waves having passed through different transmission paths arrive at an obstruction or the like with a time difference out of synchronization. Then, a signal obtained by each RAKE receiver is subjected to correction of a delay time and thereafter to RAKE composition. This arrangement has an advantage that a factor of signal deterioration caused by multi-path fading, which is conventionally a main factor of noise, can be handled as stronger radio waves by synchronizing the respective paths.
Another advantage is that having a plurality of RAKE receivers enables soft handover of receiving the same radio waves from two or more base stations, while considering them as one signal by RAKE composition. This prevents an outage phenomenon caused by frequent switching between cells at the time of handover during a call by the second generation mobile phones.
Then, in order to execute the soft handover, a mobile station (=mobile phone) needs to obtain a spread code of not only a cell of a base station (a cell whose path loss is the smallest) in which the phone itself locates but also a cell of its adjacent base station (a cell whose path loss is the second smallest). At the time of searching an adjacent base station, W-CDMA requires processing of detecting a cell having a spread code of a common pilot channel (=CPICH), which processing is called cell search. Then, when a spread code of the adjacent base station (cell whose path loss is the second smallest) is detected to establish synchronization of down spread codes, the mobile station transmits a RACH (Random Access Channel) downwards at predetermined timing, resulting in establishing synchronization of spread codes at a high speed. (Literature: “W-CDMA Mobile Communication System”, pp. 35 (ISBN4-621-04894-5))
As described in the foregoing, while W-CDMA overcomes various shortcoming of the second generation mobile phones, it involves another drawback, which is an increase in power consumption. This is because further processing which is not required in the second generation mobile phones is added to W-CDMA as described above to largely degrade facility of actual users.
First possible solution to such an increase in power consumption is omitting the above-described processing as much as possible. Out of the above-described processing, there exists cell search as the processing required even when not in communication and there have been various methods of omitting the processing.
For example, recited in Japanese Patent Laying-Open No. 6-13959 (hereinafter referred to as Literature 1) is a method of omitting detection of a frequency of an adjacent base station in using a second generation mobile phone. More specifically, the method is first measuring electric field strength of a cell in which the mobile phone itself locates and when the electric field strength is high enough, omitting measurement itself of a change of the electric field strength and electric field strength of a frequency band emitted by the adjacent base station, while when the electric field strength is low, detecting a change of the electric field strength to detect a moving state.
Literature 1, however, is a method invented on the premise that adjacent base stations use different frequencies which is a condition peculiar to the second generation mobile phones, so that the method has difficulty in application, without modification, to W-CDMA in which adjacent base stations use the same frequency.